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Regulation, 1960s style: Magnetic Amplification & Voltage Regulation No transistors or even valves are needed! This article describes how a transformer’s output voltage can be controlled using another two transformers, a potentiometer and two diodes. I recently carried out many bench experiments studying the subject of “magnetic amplifiers”. I studied some fascinating textbook concepts and methods of controlling voltages using laboriously self-wound toroidal transformers. For this particular article, I will stick to a practical theme: how some standard toroidal transformers can be used to regulate DC power (without going too much into the boring parts of the theory). I wanted to use components you can buy from places like Altronics or Jaycar, so anyone interested can easily replicate the design, whether just for a lab experiment or to make a power supply. This design delivers an adjustable 10-15V DC up to 12A without transistors, chips, microprocessors or circuit boards! We are firmly transported back to the 1960s, when silicon rectifiers were just coming onto the market, radios and computers were full of valves, and a telephone was made of black Bakelite with a rotary dial. A little bit of theory The simplest technique described in textbook literature for magnetic power control is the two saturated toroid arrangement, as shown in Fig.1. By Fred Lever Here, a pair of toroidal transformers are connected to an AC supply, with each handling one half-wave, gated by diodes D1 and D2. The power passing through the load windings (Ng) can be controlled by varying the bias on the control windings (Nc). Some very interesting waveforms are generated in doing this, as shown in Fig.2. In several separate experiments, I was able to reproduce these waveforms. The change in control bias level causes a similar change to phase control using an SCR or Triac. Fig.2(e) gives a bit of a hint of this. The curve of particular interest in the practical sense is Fig.2(g). This Fig.1 (above): the basic Magnetic Amplifier circuit, from page 457 of Benedict and Weiner’s book “Industrial circuits and applications” (see References section). Fig.2 (right): the expected waveforms in a Magnetic Amplifier, from page 458 of “Industrial circuits and applications” (see References section). 68 Silicon Chip Australia's electronics magazine siliconchip.com.au Fig.3: this is the transfer function I plotted from my experimental Magnetic Amplifier. While the control voltage range spans 80mV, the Jaycar toroidal transformers are much more sensitive, needing only millivolts to control the output over a range of amps. shows the change of load current in Ng with respect to the control (bias) current, Nc. This is similar to the bias transfer curve of electronic devices used in power control such as valves, transistors and SCRs. Fig.3 plots the transfer curve I achieved in my experiments using the hand-wound transformers. The load current could be controlled from near zero to maximum over a 40mV bias range, giving a similar result to Fig.2(g). The Jaycar transformers used in the circuit described below give a similar curve but with about twice the bias range for the 12A load. The gain of the cores is in the order of 10s of amps of load divided by milliamps of control, giving an effective gain in the thousands. sense. I could control the AC supply (a-c), over a range of current (iL) from almost zero to full load through a rheostat load, R. That was good enough to demonstrate manual control of an AC level of power by adjusting the bias level (d-c). One point of interest was if the cores were unloaded (with rheostat R open-circuit), the cores would act like air-cored chokes (no flux = no inductance) and lose control, producing the full output voltage. A hint to this is shown in both Figs.2 & 3; you will note that neither transfer curve reaches complete cut-off with no load. When you examine the circuits of industrial equipment, this situation never arises as there are extra bias Warning: Mains Voltage This project involves mains voltages which can be dangerous if not handled correctly. windings that provide a no-load flux. For simplicity, there are no extra windings in my power supply. Instead, an auxiliary circuit draws a constant current from the output, so the transformers always have a load. Fig.4 shows the circuit for a bench experiment using Jaycar MT2112 12-012V toroidal transformers. This shortform circuit can be used to verify the winding connections and draw a control transfer curve like Fig.3. The output of a 20V isolating transformer is applied to diodes D1 & D2. These drive the toroidal transformer load windings (the old secondary), which are connected in parallel series. The load winding centre tap feeds bridge rectifier BR1 to provide a Taking a practical approach The explanation of precisely what is happening inside the toroidal transformers is quite long-winded. Suffice to say that the saturating effect of the diode-guided feedback causes rapid changes in core flux that produce a ‘phase angle firing’ effect, resulting in a high effective gain. There are many books and online sources that you can peruse to understand this in more detail. An excellent mathematical treatment can be found in the paper by Brayton M. Perkins titled “Design of a self saturating magnetic amplifier utilizing high frequency excitation”, University of Arizona, 1956. You can download this from http://hdl.handle. net/10150/319332 Hooking up the Jaycar transformers on a piece of timber in the basic circuit shown in Fig.1 proved that they work using this scheme in a practical Fig.4: this is about the most basic circuit you can put together to test the Magnetic Amplifier principle. Besides the three transformers, two diodes and a bridge rectifier, you just need some meters, an adjustable load and a variable voltage to act as the bias source (which can be based on a bench supply). siliconchip.com.au Australia's electronics magazine January 2023  69 Supply Specifications Size: 420 x 265 x 200mm Weight: 15kg Output: 12-15V at up to 12A, 150W maximum Voltage regulation: ±5% Output ripple: 20mV at light loads, rising to 2V Power consumption: 300W pulsating DC output. Rheostat VR2 gives a variable load, with a voltmeter and ammeter connected to it. The colours of the transformer windings are shown in Fig.4. The control windings of the toroidal transformers (the old primary) are connected in series and to a DC lab supply of about 12V for bias. This needs to supply positive and negative bias voltages to swing the load windings over the entire range. As I only have a single polarity adjustable supply, I used a wirewound 100W resistor with a centre tap, plus wirewound 300W potentiometer VR1 to form a bridge-style circuit to give both polarities. During testing, the voltages applied were in the range of −1V to +1V at up to 500mA. The load rheostat I used (VR2) was rated at 500W and could handle load voltages up to 20V with currents of up to 20A. All the meters I used are true-RMS responding. Once set up, the output levels can be plotted against the control bias voltage. I adjusted the load rheostat to get 12V across it for loads applied in 1A steps by varying VR1. This gives a plot similar to Fig.3. If you can’t control the output as expected, that suggests a connection-­ phasing problem. There are 12 connections to the toroidal transformers that all need to be made correctly, as per Fig.4; one wrong connection will result in incorrect operation. Practical power supply design Moving on to the practical power supply, the short form circuit is expanded to include components to reduce the ripple on the DC output and provide the necessary controls and protection. The whole circuit is shown in Fig.5. All the required sections of a 1960s era supply are included in the finished design, specifically: 1) an unregulated source of AC 2) a power regulating control device 3) an independent reference supply 4) an error amplifier and correction signal 5) a rectifier & ripple control device 6) stability control to provide any transient damping and correct hunting Some wonderful old textbooks exist that clearly explain some of these points and are well worth reading, such as “Industrial Electronics” by Gullicksen and Vedder (1935); see the References section at the end of the article for more. Expanding on these: For this general-purpose bench supply, isolation from the mains is required, and a transformer with a nominal output of 24V AC at 12A can provide this. This sets the limit for the maximum output current of the supply. This transformer, T1, needs to provide a minimum of 20V AC at full load to give enough headroom for the power control device to deliver 15V DC. I used a transformer rescued from a discarded 300W UPS in this supply. Other transformers can be used, either toroidal or E-core, so long as they can supply the voltage and current required. #2 The regulation control devices in this supply are a pair of Jaycar MT2112 toroidal transformers, T2 and T3. This pair can handle about 20A of load current in this circuit arrangement, having an individual secondary rating of 12A with the secondaries connected in parallel. Each transformer handles onehalf wave of the AC power as guided by bridge BR1, so they are operated well within their ratings. Using devices that you can buy off the shelf removes the frustration of sourcing toroidal cores and copper wire, and the pain of winding them. #3 The reference supply could be any circuit that provides an adjustable 10-15V DC into a nominal 100W load. I experimented with various sources such as an independent bench supply, a battery of AA cells, a magnetic saturable reactor, a zener diode supply, #1 Fig.5: this more complete Magnetic Amplifier circuit gives a practical, usable adjustable voltage source for powering various circuits and doing things like charging batteries. While it has some limitations compared to the valve-based adjustable supplies back in the day, it has a certain elegance. Its simplicity means that such a supply would have been considerably cheaper to produce. 70 Silicon Chip Australia's electronics magazine siliconchip.com.au a 7815 regulator IC based supply and an unregulated 15V DC supply using a small Jaycar MT2002 transformer and a bridge rectifier. The most practical configuration that a home builder can easily reproduce is the last one, once again using readily-available parts. The stability and accuracy of this reference largely determine the performance of the overall supply. This corresponds to the portion of Fig.5 that includes T4, BR3 and VR1. It works well enough for many practical jobs, such as testing automotive 12V parts and supervised lead-acid battery charging. Performance could be improved with extra components to stabilise the voltage across the 2200µF filter capacitor, eg, a zener diode or an integrated regulator. #4 The error amplifier circuit in this supply is the simplest kind possible. The terminal voltage of the supply is applied to one end of the toroidal transformer control windings, +SENSE, and the reference voltage connected to the other end, +REF. Any differential between the two voltage levels causes a bias to be applied to the control cores. The reference supply is made variable from 10-15V, which becomes the panel control to set the voltage. The phasing of the control windings and the connection to the external circuits is critical; only the connection as shown on the circuit will work correctly. In a steady state, the differential voltage parks the toroidal transformers at a point on the transfer curve. With any disturbance such as moving the set voltage control or a change in load impedance, the differential voltage shifts the operating point on the curve and equilibrium is restored to suit the disturbance once the system’s time constant elapses. #5 The AC-to-DC rectifier, BR2, is a straightforward rectification bridge. An LC low-pass filter is formed using inductor L1 and a large 15,000µF capacitor provide ripple control. This is a more practical solution than just using a huge capacitor bank, with the benefit of a lower phase lag effect on the transient response, which could otherwise lead to instability. The capacitor used should be a proper low-ESR power supply filter capacitor. Up to about 10A can flow through it depending on the capacitor siliconchip.com.au The internals of the finished supply – it’s bulky but simple. The front panel and base plate are Earthed for safety, while mains-rated terminal blocks are used to make the connections. Australia's electronics magazine January 2023  71 Parts List For the test rig shown in Fig.4 1 24V output mains transformer, ideally at least 300VA (T1) 2 12-0-12V toroidal mains transformers (T2, T3) [Jaycar MT2112] 2 400V 35A bridge rectifiers (D1, D2, BR1) [Jaycar ZR1324] For the complete supply shown in Fig.5 1 24V output mains transformer, ideally at least 300VA (T1) 2 12-0-12V toroidal mains transformers (T2, T3) [Jaycar MT2112] 1 15V output mains transformer, ~15VA (T4) [Jaycar MM2002] 2 400V 35A bridge rectifiers (BR1, BR2) [Jaycar ZR1324] 1 400V 6A bridge rectifier (BR3) [Jaycar ZR1360] 1 20A+ diode (D1) [Jaycar ZR1039] 1 60mm 12V DC fan (FAN1) 1 12V lamp (LAMP1) 1 ~73mH 12A choke (L1) 1 20V FSD moving coil panel voltmeter [Jaycar QP5020] 1 20A FSD moving coil panel ammeter [Jaycar QP5016] 1 2A mains circuit breaker (CB1) 1 15A mains circuit breaker (CB2) Capacitors 1 15,000μF 40V 23A electrolytic power supply filter capacitor 2 2200μF 25V electrolytic [Jaycar RE6330] Resistors 3 100W 10W 10% wirewound (paralleled to give 33W 30W) [Jaycar RR3364] 1 39W 5W 10% wirewound [Jaycar RR3264] 2 47W 1W 5% carbon film [Jaycar RR2542] 2 150W 1W 5% carbon film [Jaycar RR2554] 1 100W wirewound potentiometer (VR1) value, choke size and load. Large standard electrolytic capacitors will work but will get hot and have a shorter life than a power supply capacitor. Any capacitor that does not have screw connections is not the best permanent choice. The capacitor I used was rated at 15,000µF, 40V DC with a ripple current of 23A. The 73mH, 15A filter choke I used is not an over-the-counter item at Jaycar! A functional unit can be wound using the stack of E and I laminations from a discarded transformer. My unit had around 40mm2 of core rated at about 120W, and I crammed 100 turns of 12A wire into the window to achieve 73mH. The inductance value is not critical; the trade-off is in physical size. I would have liked at least 250mH, but that would have taken a 300W size lamination stack and 150 turns of 15A wire. What I used is good enough for the job. I stacked the laminations interleaved but not air-gapped; the iron saturates on full load, giving the operation known as a ‘swinging choke’. 72 Silicon Chip The transient response of the supply is determined by the time lags inherent in the circuit. All of the gain in the comparator section is contained within the control toroidal transformers and is just sufficient to give millivolt-­level regulation. No anti-hunt or phase lead/lag techniques external to the comparator are needed to modify the transient response for stability. In addition to points #1-#6, a few other items are needed for a practical supply, such as terminals, meters, and overload or fault shut-off protection. In the supply described here, I included CB1, a 2A AC circuit breaker on the input that doubles as a power switch; CB2, a 15A AC circuit breaker on the DC output that doubles as a load switch; a panel light (LAMP1) to indicate life; and a pair of panel meters to indicate voltage and current levels. The meters can be just about any type that is available with the ranges required. You could replace the breakers with switches and fuses of similar ratings, but breakers are easier to reset. I used junked units from old electrical switchboards. #6 Australia's electronics magazine The components can be mounted in a cabinet that could be re-purposed or made from scratch. I mounted a heatsink inside that carries parts that will get hot such as the bridge rectifiers and the 33W ballast resistor. The heatsink can be anything made of metal of about the same size used here. Fan FAN1 is mounted in the cabinet to push air in over the heatsink and through the cabinet. This could be a 12V DC powered fan, or a mains-­ powered type around 60mm rescued from another device. General operation The mains is isolated and reduced to 24V AC at no load by transformer T1. Bridge BR1 may look to be connected strangely, but functions as a diode guide to gate toroidal transformers T2 and T3 with alternate halfwaves from T1. Toroidal transformers T2 and T3 are the power control devices, with their secondary inductance varied to regulate the output voltage. The cores need to have a high inductance on low load and a falling inductance as load current increases. This is accomplished by applying a bias current to the control windings (formerly primaries) of T2 and T3. Rectifier BR2 converts the controlled AC voltage to DC with a large ripple content, which is then applied to choke L1. This choke, combined with the 15,000µF capacitor, provides a low-pass filter to remove the 100Hz ripple. It is known as a “swinging choke” since it saturates as the load increases and its inductance falls to a lower value. Diode D1 is strapped across the outgoing rail to assist CB2 to trip if a reverse polarity is applied back into the output terminals, such as an incorrectly connected battery. The 33W resistor provides a minimum load to the toroidal transformers so that with no external load, some flux is generated in the toroidal windings, and start-up inductance is assured. The panel lamp and the cooling fan are also fed from this point to add to the minimum load current, resulting in around 0.5A. The cooling fan, FAN1, is run at a reduced voltage due to its series resistor. This limits the maximum voltage applied when the supply is set to 15V, especially as it has a high load ripple. Under this condition, without siliconchip.com.au the resistor, the fan coil could experience up to 18V. The fan runs at a slow speed on a low voltage setting and speeds up in proportion to voltage setting and load, with 12V applied when there is a high-current load and the output is set to 15V. Output ripple The output ripple level varies, going up as the load rises and is predominately 100Hz. With the values of L and C used, at 100Hz, the inductive reactance of the choke is about 40W and the capacitor reactance is about 0.1W. Thus, on a low load, the 100Hz component is attenuated by a factor of about 400 (40W ÷ 0.1W). The resulting ripple is in the 10s of millivolts. As the load current rises, the choke saturates and its inductance falls. This causes the loading effect at 100Hz to reduce and, at full load, its inductance is about 10% of nominal, giving a reactance of about 4W. The ripple attenuation factor is then approximately 4W ÷ 0.1W = 40 times, giving ripple levels of volts on top of the DC. This could be reduced by using a physically larger filter choke. If the reference voltage is lost or too low (<8V), the toroidal transformers may lose control and turn fully on. After the usual mains safety checks, the first power-up of the circuit can be via a reduced supply such as from a variac or with a light bulb in series with the mains supply. Applying power, you will note that the voltmeter swings up to the set voltage, and the circuit breakers should not trip. Then the supply is ready for testing. The voltage control should swing the output voltage between about 11V and 15V. Apply a load and the voltmeter will dip, then rise back close to the set voltage. Shed the load and the voltmeter will swing high momentarily and then settle close to the set value. If a short circuit is applied, the ammeter will smack hard over past 20A and then, depending on the tripping curve of the circuit breaker, a few seconds will elapse until it trips off. Supply waveforms Noting the difference between waveforms at no load and full load can give insight into how the control scheme works. No part of this supply circuit is connected to mains Earth except for the metalwork. Thus, an oscilloscope’s ground lead and probes can be connected anywhere on the low-voltage circuitry to examine the waveforms at any point with no fear of smoking Earth leads! Scope 1 shows the AC voltage from Powering it up Apart from getting the phasing of the toroidal transformer windings correct, there are no mysteries. If the connections are incorrect, the output might be the full uncontrolled voltage, a low voltage or just not work. If the 33W ballast resistor is not fitted, the transformers will simply operate like air-cored chokes and give the full output voltage. siliconchip.com.au The supply is mounted in a wooden cabinet. The heavy electrics are also bolted to an internal Earthed steel chassis. The cabinet is screwed to this chassis and this also secures the front panel. Australia's electronics magazine January 2023  73 Scope 1: the output of the mains transformer feeding this circuit is a distorted sinewave similar to the incoming mains waveform. Scope 2: the voltage across control transformers T2 & T3 under a low load condition. The spikes are due to core magnetic hysteresis. Scope 3: the waveform delivered to bridge rectifier BR2 when the output is not drawing much current. the isolating transformer, which has minor distortion. Scope 2 & 3 show the no-load AC voltage across T2 & T3 and at the junction of T2 & T3, respectively. Scope 4 & 5 repeat this but at full load. With no load, the toroidal transformer reactance is high, and a large portion of T1’s output voltage appears across them, with the remainder fed to the output. The toroidal transformer reactances are low at full load. Only a small voltage drop remains across them; the bulk of the sinewave is transferred to rectifier BR2. Scope 6 depicts the AC voltage across BR2 (yellow) and half-wave positive rectified output (cyan) with no load, while Scope 7 repeats this for the full-load condition. Scope 8 shows the output ripple (yellow) with no load, measuring ~60mV peak-to-peak, with the voltage across BR2 in cyan, while Scope 9 shows the same but at full load, giving about 4V peak-to-peak ripple. Scope 10 shows the transient response of the supply when switching from no load to full load and back at 1s/div and 2V/div. The voltage dip and overshoot is about 8V, with a recovery time of about one second. The voltage regulation on load is within the ripple level; an average-­ reading panel meter interprets this as a fall of 0.5V, while an RMS-­responding meter interprets as a drop of 0.25V. A peak-responding meter shows a rise of 0.5V (due to the ripple), so take your pick! Even just as a lab experiment, it would be prudent to mount the heavy isolating transformer T1 and control transformers T2 & T3 on a decent base like a sheet of MDF or plywood with an Earthed aluminium sheet adhered to the top for safety. As mentioned earlier, the phase and order of winding connections is critical. It so happened that the correct order of connections on my Jaycar transformers followed the notions of ‘starts’ and ‘finishes’ of the windings in order around the cores. The heavy windings are colour-coded as to where they start (a dot symbol on the drawing) and finish (no dot). The control windings (primaries) use all blue wires but emerge in a uniform order, from start to finish. Unfortunately, this means that while you can easily figure out how to wire them correctly in series, the polarity of the bias voltage connection is not obvious. So if the circuit doesn’t work as expected, the first thing to try is swapping the polarity of the control voltage to those windings. For convenience, bridge rectifiers BR1 and BR2 can be bolted to a piece of metal acting as a heatsink. Many of the winding connections join there, as you can see in my photos. You could choose to build the ‘lab exercise’ circuit shown in Fig.4 or progress to the power supply of Fig 5. This being a mains-powered circuit, you have to be careful how you wire it up to ensure it is safe. Follow my photos and ensure all the following steps are taken: • Use 10A mains-rated wire for all the mains connections in the correct colours: green/yellow striped for Earth, brown for Active and light blue for Neutral. • Insulate all exposed points at Active or Neutral potential with heatshrink tubing or similar insulating material (don’t use electrical tape except as a temporary measure). If using crimp connectors for the mains wiring, ensure they are appropriately sized and are the insulated type (or add heatshrink tubing over the top as insulation). • The incoming Earth wire must go straight to a substantial lug making good electrical contact with the metal base plate. Other Earth wires can run from this point to any other metal panels (eg, the front panel and/ Scope 6: the AC voltage across BR2 (yellow) with a light load, plus one half of the rectified waveform (cyan), taken from the positive side of the bridge only. Scope 7: the AC voltage across BR2 (yellow) at full load, plus one half of the rectified waveform (cyan), taken from the positive side of the bridge only. Construction advice This is more of an experiment than a project. Despite that, I have included a parts list (in case you want to try the experiment yourself) and some basic guidance on how to build such a supply. 74 Silicon Chip Australia's electronics magazine siliconchip.com.au Scope 4: the voltage across control transformers T2 & T3 at full load. They no longer drop much voltage across much of the mains waveform. Scope 5: the voltage applied to bridge rectifier BR2 at full load. This looks an awful lot like a Triac phase control waveform! or lid). There is no need to make Earth connections elsewhere on the supply, except perhaps if you wish to provide a front-panel Earth binding post. • Ensure that there are no exposed mains-potential metal contact points on any mains sockets or switches (including when the switch is in either position). • Use cable ties to connect mains wires together close to any connection point. This is so that if one wire breaks loose, it is held together with the rest of the bundle and can’t move to contact any low-voltage wiring or exposed metal. • Use a mains-rated terminal block, ideally bolted to the base, to connect the incoming mains wires to the mains transformer. Place a sheet of insulating material such as Presspahn, cut larger than the terminal block, between this and the Earthed base. • Ensure proper separation between all mains wiring and all isolated, low-voltage wiring. It’s best to keep all the mains wiring in a separate chassis section, away from the rest. One thing to note in my photos is the lack of cable ties on each side of the terminal block that joins the incoming mains wires to the transformer primary (I mounted this on a bracket attached to the base to save space). I corrected this omission after taking the photos. Scope 8: the ripple at the unit’s output (yellow) at no load, with the input of the LC filter (cyan). The yellow trace is less than 50mV peak-to-peak (p-p), while the cyan waveform is ~25V p-p. Scope 9: the ripple at the unit’s output (yellow) at full load, with the input of the LC filter (cyan). The yellow trace is around 2.3V RMS, while the cyan waveform at about 30V p-p. siliconchip.com.au Two different versions For the short form circuit (Fig.4), the bridge output can be terminated in the ballast resistor. You can then connect a suitable load bank (resistors or lights) directly to the rectifier output with flying leads. Connect measuring instruments (volt/ammeters) as needed. The reference supply can be a bench supply arranged so that voltages of either polarity can be applied to the toroidal transformer control windings. That may be all that some people wish to do to experiment. There is no reason that cheaper, lower-current toroidal transformers cannot be used for such a demonstration; the main advantage of the specified transformers is that it saves a lot of time and effort compared to winding your own. However, Fig.5 can be built into a working, practical supply, as shown in my photos. I expanded the floor plan to add in the filtering components and the reference transformer, then packed Australia's electronics magazine the rest into the rear of the enclosure and the front panel. Since most people who decide to build this version will have differently-­ s ized enclosures, it’s hard to give highly detailed assembly instructions. Look at my photos, decide how you can adapt the layout to your enclosure and start mounting and wiring the bits. Just make sure you follow the safety advice above. The physical size of the enclosure will depend on the parts used. I wound up with a 400mm wide unit with 240mm of depth and 200mm of height. The enclosure I made was a composite of plywood and steel sheets. The steel sheets are all connected to Earth wires for safety. The front panel has an angled metal section to carry the meters and voltage control, also Earthed. The terminals and circuit breakers are mounted on a ply section. The floor is a plywood sheet with a metal sheet laid over it, Earthed as described above. The heavy parts are bolted to the floor, with the remainder screwed to the rear of the front panel. The front panel is mounted on hinges, has the operator controls and load terminals and swings down once released by removing the plywood cabinet. All the essential details are shown in my photos. Happy experimenting! References 1. Book: Benedict and Weiner, 1965, “Industrial circuits and applications”, Prentice Hall, NJ. 2. Paper: Brayton M Perkins, 1956, “Design of a self saturating magnetic amplifier utilizing high frequency excitation”, University of Arizona (http://hdl.handle.net/10150/319332). 3. Book: Gullicksen and Vedder, SC 1935, “Industrial Electronics”. Scope 10: the transient response from light load to full load and back. The regulation is good, but there is more ripple on the output under full load, and the response time is slow (~0.5s). January 2023  75